Ember model hasMany not working

I'm running into an issue with some very simple Ember setup. I have two separate models, appointment and item. My appointment model has a property, appointmentBins, which hasMany('item'). However, the array is always coming back empty and I have no idea why. Here are the relevant models:

// appointment model
let attr = DS.attr;

export default DS.Model.extend({
  // ...
  appointmentBins: DS.hasMany('item'),
  confirmed: attr('boolean'),
  specialInstructions: attr('string'),
  // ...
}); 


// item model
let attr = DS.attr;

export default DS.Model.extend({
  title: attr('string'),
  userComments: attr('string'),
  internalComments: attr('string'),
  // ...
});

And here's the appointment serializer that I'm using:

export default DS.ActiveModelSerializer.extend(DS.EmbeddedRecordsMixin, {
  isNewSerializerAPI: true,
  attrs: {
    appointmentBins: { serialize: 'ids', deserializer: 'records'},
    // ...
  }
});

And finally, here's the response from the network call:

appointment: {
  appointment_bins: [,…]
    0: {id: 27, title: "", user_comments: null, internal_comments: null, user_id: 23, item_category_id: 10,…}
    1: {id: 28, title: "", user_comments: null, internal_comments: null, user_id: 23, item_category_id: 10,…}
    2: {id: 29, title: "", user_comments: null, internal_comments: null, user_id: 23, item_category_id: 10,…}
    3: {id: 30, title: "", user_comments: null, internal_comments: null, user_id: 23, item_category_id: 10,…}
  confirmed: false
  id: "f2eb7da8b2b31f2ebad0"
  special_instructions: ""
  // ...
}

I have no idea what I'm missing - this doesn't seem to be anything particularly complex, but I just can't parse the returned array inside the appointments model. Help greatly appreciated!